1. Field of the Invention
Embodiments described herein relates generally to a power conversion apparatus that produces alternating-current power from direct-current power and to a power conversion apparatus that produces direct-current power from alternating-current power, such as a photovoltaic power generation system, a fuel-cell power generation system, or a secondary-cell energy storage system.
2. Description of the Related Art
In a photovoltaic power generation system, a power conversion apparatus generally converts direct-current power generated by photovoltaic cells into alternating-current power and supplies the alternating-current power to a linked system such as a power system, a distribution system, and a load. Since the characteristics of the photovoltaic cells vary, depending on insolation or temperature conditions, a maximum power point tracking method has been employed to constantly obtain the maximum power. For example, as disclosed in patent document 5, the voltage or current of a photovoltaic cell is controlled to the optimum point at which the voltage or current varies, depending on insolation or temperature conditions.
As for a circuit configuration of a power conversion apparatus, use of a multilevel conversion circuit, such as a three-level inverter as shown in patent documents 1, 3 and 4, suppresses harmonic currents in an input and an output and realizes the miniaturization of filters provided at the input and output and an improvement in the efficiency of the apparatus.
Furthermore, patent document 2 has described the comparison between conduction losses and switching (SW) losses in a three-level inverter and a two-level inverter.